The impact of the target thermal structure on seeker and sensor acquisition is well known.
Night vision systems are used extensively for military and security purposes. These include thermal imaging cameras and ATR (automatic target recognition) systems that automatically classify targets by their thermal signature.
There are two principle approaches:
1) Detecting infrared radiation, which is a form of energy emitted by all objects regardless of the ambient light conditions, using an infrared camera.
2) Intensifying the small amount of light present, even at night, from the stars and the moon.
Most objects have a radiated temperature either higher or lower than their background. Even if the radiated temperature differences are less than a degree, they can be detected. If there is no difference between the temperature of an object and its background, the object cannot be seen by a thermal imaging night vision system or by infra red based heat seeking missiles.
Thermal imaging can see through light fog and mist and, more importantly, through most camouflage. The fire control systems of most armored vehicles have night vision, usually thermal imaging.
Today, solutions based on active countermeasures against infrared detection and tracking can be combined with passive stealth measures; these include infrared jamming (i.e., mounting of flickering infrared radiators to confuse the tracking circuits of heat-seeking missiles) and the launching of infrared decoy flares.
Usually, targets are easier to identify at night, because their radiated temperature is hotter than their background. Some targets, such as tanks and APCs, have internal temperature variations that form visible patterns. The shapes of the hottest vehicle parts, such as engines and exhausts, appear bright. Objects with a medium temperature, such as the warm tracks, appear dim. Objects with a cool temperature, such as the cool hull, appear black.
The sources of infrared energy are solar heat, fuel combustion heat, frictional heat, and reflected radiance.
Solar Heat—comes from the sun and affects the exterior surface of objects. The heating highlights the outline of the object, providing recognition cues to the viewer, which are usually similar to the overall appearance of the target. These shape cues are recognizable out to medium ranges (800 to 1,200 meters) and can be detected at long ranges (2,000 meters). Since the sides of vehicles have more defined contours, side views are usually easier to recognize than the front views.
Fuel Combustion Heat—comes from operating engines. The heat is conducted to the surfaces of the surrounding engine compartment. Because engine compartment temperatures reach up to 200 degrees F., the surfaces of these compartments radiate features that can be detected.
Frictional Heat—produced by the moving parts of vehicles. Its heat is less intense than the high temperatures from the engine combustion. Frictional heat is generated only when the vehicle is in motion and provides long-range cues to classify the vehicle as wheeled or tracked.
Reflected Radiance—smooth, glossy surfaces, such as windshields and glossy, painted fenders, reflect radiation images from other sources. These reflections can produce odd images.
A gun tube is visible when recently fired, as the gun tube is heated up. Similarly, the transport mechanism becomes warmer and more visible.
All Infrared (IR) direct threat weapons require line of sight (LOS) to be established prior to launch and the in-flight missile must maintain LOS with the target heat source until impact (or detonation of the proximity fuse). IR missiles require the operator to visually detect the target and energize the seeker before the sensor acquires the target. The operator must track the target with the seeker caged to the LOS, until it is determined that the IR sensor is tracking the target and not any background objects. In addition, semi-automatic homing IR missiles detect the missile and navigate by IR sensing of the target. The IR sensor is also susceptible to atmospheric conditions (haze, humidity), the signature of the aircraft and its background, flares, decoys, and jamming.
Man Portable Air Defense Systems (MANPADS) pose a serious threat to aircraft at present. Rather than simply providing a second bright IR source in an attempt to draw an approaching missile away from a targeted aircraft, Directed Infrared Countermeasures Systems (DIRCM) use beams of light produced by a variety of means, such as flashlamps, to exploit knowledge about the design of reticle-scan MANPADS seekers to defeat their homing mechanisms. In many MANPADS, a reticle within the seeker causes pulses of light from the target aircraft to “shine” on the missile's infrared detector. The IR detector senses the IR radiation and sends an electric signal to the guidance package, which determines the target location and allows the missile to track the target aircraft's location and movement through the sky. By shining a modulated light towards the seeker, an IRCM system provides the infrared detector with extra “false” data, which deceives or “jams” the missile, causing it to miss its intended victim.
Viewing targets during normal and limited visibility requires gunner training on thermal target recognition, identification, and engagement. The gunner or ATR must interpret unusual images with the night tracker. These images, called thermal target signatures or infrared target signatures, are different from the images seen in the day tracker. Targets stand out in these infrared images and can be recognized at long ranges on a clear night and at reduced ranges during limited visibility. However, the recognition task requires trained and experienced gunners so the task may not be simple.
Other Terms that May Enhance Detection by Thermal Viewer and Countermeasure by this Patent
During rain or snow, background objects and frictionally heated and solar-heated target features lose heat. Frictional heat loss is caused by water and mud collecting on the tracks, wheels, and other transport system parts. Engine compartment and exhaust temperatures remain high. Landmarks, such as trees, trails, and contour features, are often lost. The loss of heat in background objects reduces scene clutter, such as trees and rocks, and can increase target detection. In this type of situation the system ability of camouflage (stealth) is well needed.
In a target-rich environment on a dry, clear night, high-confidence identification requires a thermal image of such features as road wheels, turret shapes, gun tube and exhaust location. Thus, target recognition is a difficult task that requires an expert, so any change of heat signature will create chaos.
In order to help those who are not experts, Automatic Target Recognition (ATR) has been developed. Automatic Target Recognition is an application of computer vision to identify targets (such as tanks or airplanes) in an image. The process involves obtaining essential features (edges, ridges, corners) from each local geographical area in the image and comparing it to the stored templates of known targets. If a match is found, then the target is declared to be present at that location.
Environmental monitoring, earth-resource mapping, and military systems require broad-area imaging at high resolutions. Many times the imagery must be acquired in inclement weather or during night as well as day. Synthetic Aperture Radar (SAR) provides such a capability. SAR systems take advantage of the long-range propagation characteristics of radar signals and the complex information processing capability of modern digital electronics to provide high-resolution imagery. Synthetic aperture radar complements photographic and other optical imaging capabilities because of the minimum constraints on time-of-day and atmospheric conditions and because of the unique responses of terrain and cultural targets to radar frequencies. Thus, synthetic aperture radar technology can provide reconnaissance and targeting information to military operations
When using automatic target detection with SAR imagery, the matching process is very difficult because of the noisy clutter background. Additionally, the target pose (orientation) and scale variation (due to imaging altitude) add to the complexity of the process.
Helicopters and other rotorcraft provide slightly different problems than land vehicles. In order to confuse thermal heat seeking missiles and avoid hitting and detection, the choice today is to throw flares that burn at high temperatures and generate a dominating infra-red signature. However, it is not possible to use such flares near the ground when forces are deployed or when there is need to evacuate wounded soldiers, as they can hurt the forces. Also, there exist smart missiles systems which can discriminate between flares and the real target.
Accordingly, there is a long felt need for a system to permit objects to remain hidden from thermal detection devices, and it would be very desirable if this system can operate in a variety of different ways.